1. Field of the Invention
The present invention relates to chemical sensor coatings. More specifically, the present invention relates to processes for covalently attaching and patterning sensing films selectively onto the surface of a sensor using photopolymerization.
2. Description of Related Art
The photopolymerization of acrylates has been intensively studied over a period of decades. Polym. Int. 1998, 45, 133; Mater. Sci. Technol. 1997, 18, 615. Basic version of the process have been studied and/or patented for various applications, including the UV-curing of acrylate coatings on optical fibers (see U.S. Pat. No. 5,567,794). In addition to these, applications such as imaging processes and information recording (U.S. Pat. Nos. 5,374,184, 4,050,936), photo-resist processes (U.S. Pat. No. 3,660,088), polymerizing liquid crystal structures (Langmuir, 1999, 15, 631), and medical applications (Proc. Natl. Acad. Sci. USA, 1999, 96, 3104) have been widely studied and published.
Another area of acrylate photopolymerization receiving attention is the manufacturing of chemical sensing films. Chemical sensing film coatings are currently manufactured using a variety of methods, including the direct coating of polymers or molecules onto a sensor surface (Chemtech 1994, 24, 27), molecular self-assembly (Electronics Letter, 1997, 33, 1651), solution chemistry to covalently immobilize polymeric molecules onto the sensor surface (Langmuir 1998, 14, 1505), and poly-electrolyte layer-by-layer deposition (Sensors and Actuators B, 1997, 45, 87). These methods generally result in the deposition of a sensing film layer onto a surface. In practice, however, films deposited using these methods suffer from problems that decrease their usability.
Among these problems is the lack of effective surface bonding of the film with the surface of its substrate. This problem renders many films susceptible to being stripped from the surface by aging, environments, or physical contact with other objects. This is a significant disadvantage since some such physical contact is incidental to normal use of the sensor. Such stripping renders the sensor less sensitive, and thus less useful. In addition to this problem, the phenomenon of polymer dewetting may cause instability in the resulting film and the formation of xe2x80x9cislandsxe2x80x9d of polymer which affect the sensitivity and accuracy of the sensor.
Other problems stem from the tendency of some films to scatter or absorb light. In many transduction-based sensor systems, a film must be very thin and transparent in order to allow proper sensor function. If the film chosen is not sufficiently thin and transparent, light is lost, thus rendering an optical sensor coated by the polymer inefficient and inaccurate in its function.
Solution-grown films demonstrate improved stability over many of the currently-used techniques for thin-film fabrication. Unfortunately, most such solution chemistry methods require several days of deposition for completion. Such lengthy fabrication methods often add inconvenience and expense to the production costs of chemical microsensor films. Further, many sensors are not robust enough to undergo the lengthy processing, thus further reducing the utility of these methods.
In addition to these problems, many applications require sensors capable of distinguishing multiple chemicals. Current sensing approaches embrace this xe2x80x9cdog""s nosexe2x80x9d approach to chemical sensing, but have difficulty providing sensors having a sufficient number of different elements for binding each different target molecule. Many sensors are incapable of this, and would thus be improved if they were able to sense multiple chemicals at once. One of the greatest difficulties is patterning differing films onto the small elements such as those used in miniaturized, multielement devices. Patterning chemically distinct films onto the sensing surfaces of the different sensor elements could confer such ability. Currently, however, such patterned surfaces are very difficult to inexpensively produce with accuracy, thus resulting in expensive sensors when they are successfully produced. It would clearly be an improvement in the art to provide a method for conveniently and simply producing patterned sensing films and sensors with patterned sensing films.
It should also be noted that alternative mechanisms for chemical sensing on a film are needed to broaden the art and give alternatives to users with needs which are novel or unmet by current technology. One technique currently unsuitable for chemical sensing is molecular imprinting. Molecular imprinting is a technique used to produce powders imprinted by a template molecule. These materials are currently primarily used to separate the template from other substances. In one major application, these powders are used to pack chromatographic columns for use in separations. The methods currently known and practiced, however, do not teach thin, substantially transparent, imprinted films that would be suitable for use in sensing microsensor film applications.
Accordingly, a need exists for a photopolymerization method that produces sensing films that are covalently-attached to a selected surface in seconds. A need further exists for a photopolymerization method that produces cross-linking of reagents upon polymerization, thus forming a highly stable film. A need also exists for a photopolymerization method that allows sensors/surfaces to be patterned by coating them with different sensing films. Finally, a need exists for a photopolymerization method that is suitable for the production of a molecular-imprinted film suitable for use as a chemical microsensor. Such methods and devices are presented herein.
The apparatus and methods of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available methods of fabricating sensing films, apparatuses comprising sensing films, and coating solutions for forming sensing films. Thus, it is an overall objective of the present invention to provide such methods, apparatuses, and solutions which exhibit improvements over the current art.
To achieve the foregoing objective, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, a method for fabricating sensing films is provided. According to one configuration, the method of attaching a chemical microsensor film to an oxide surface may comprise the steps of pretreating the oxide surface to form a functionalized surface, coating the functionalized surface with a prepolymer solution, exposing the prepolymer solution to an analyte for molecular imprinting and polymerizing the prepolymer solution film with ultraviolet light to form the chemical microsensor film. The method may additionally include the step of rinsing away un-polymerized prepolymer solution and also may include the step of repeating the method with a chemically distinct prepolymer solution and/or analyte template.
In the invention, the surface chosen as the substrate for the chemical microsensor film must be an oxide surface. This is so in order to accommodate many different transduction approaches such as surface acoustic wave (or xe2x80x9cSAWxe2x80x9d), optical waveguides, fiber optics, and electrical transduction (such as indium-tin-oxide, which is conducting). In addition to surfaces originally including an oxide layer, surfaces for use in waveguide applications or other evanescent transduction approaches without a suitable oxide layer could be made suitable for use by attaching an intermediate layer which would have a surface oxide layer. This could be accomplished by applying a thin layer of SiO2 to the surface of the material to provide the needed free hydroxyl groups. This would be useful in applications where higher index waveguiding materials are used as the sensor substrate.
The step of pretreating the oxide surface to form a functionalized surface may comprise treating the oxide surface with a silane compound. This silane compound may be selected from the group consisting of (3-Acryloxypropanyl)trichlorosilane and 7-octenyltrichlorosilane.
The step of coating the functionalized surface with a prepolymer solution may be accomplished using a method selected from the group consisting of spin-coating, spray-coating, and dip-coating.
The prepolymer solution ma y comprise monomers of the intended polymer alone. The prepolymer solution may alternatively comprise segments of the intended polymer alone. In other alternatives, the prepolymer solution comprises both monomers and polymers of the intended polymer. In some of these, the monomer is a cyclodextrin. The cyclodextrin monomer may be selected from the group consisting of 2-Per-(2,4,6-trimethyl)benzoxy-6-per-(12-acrylaminododecanoxy)-gamma-cyclodextrin, 2-Per-(2,4,6-trimethyl)benzoxy-6-per-(12-acrylaminododecanoxy)-alpha-cyclodextrin, and 2-Per-(2,4,6-trimethyl)benzoxy-6-per-(12-acrylaminododecanoxy)-beta-cyclodextrin, and 5-tetrafulvalenylmethoxy-1-pentene.
In other versions of the method of this invention, the monomer is selected from the group consisting of a wide range of host molecules whose specificity for a xe2x80x9cguestxe2x80x9d or target molecule can be modified and optimized. This family of molecules includes calix-n-arenes, cyclophanes, and cyclodextrins. Such monomers may generally be varied by altering the R sidechain groups located proximally to the site or region where the monomer interacts with the target analyte. Though the instant invention focuses primarily on cyclodextrins (alpha, beta, and gamma with modified functionality on the side of the rims of the bucket-shaped core), similar methods may be practiced with calix-n-arenes, cyclophanes, and other molecules with polymerizable units (e.g. vinyl groups).
As noted above, the prepolymer solution may further comprise template molecules which become releaseably trapped in the final polymer after photopolymerization. After photopolymerization, the template may be removed by methods known in the art, leaving a pit in the surface of the resulting polymer film in the size and shape of the template. As a result, the film becomes attractive to template molecules or those of similar size and/or shape. Thus, either the target analyte or an analog may be used as a template molecule.
A wide variety of template molecules may be suitable for the practice of the invention. The template molecules may first be selected from the group consisting of TNT (trinitrotoluene), TNB (trinitrobenzene), DNT (dinitrotoluene), and TTF (tetrafulvalene). Sensors tuned to identify these target molecules could, for example, be used in the detection of land mines or other similarly-composed land mines. In other applications, organophosphorous molecules including chemical warfare agents such as DMMP would be suitable templates. TTF devices may be included to detect DNT and TNT by forming change transfer complexes. Other suitable organophosphates could include pesticides and insecticides.
Other target molecules suitable for use as templates could include common industrial chemicals such as arenes, chlorinated hydrocarbons, etc. Such sensors would prove useful in environmental sensing of pollutants, as in ground water testing, or for process monitoring in the chemical industry. Additionally, volatile organic compounds are a useful target.
The methods of the invention using template molecules, referred to as xe2x80x9cmolecular imprinting,xe2x80x9d may further comprise the step of extracting the template molecules from the chemical microsensor film after the step of polymerizing the prepolymer solution with ultraviolet light to form the chemical microsensor film.
After application of the prepolymer solution to the surface to be patterned with sensing film, a polymer film is formed by exposing the prepolymer solution to radiation. This step of polymerizing this prepolymer coating may be accomplished with ultraviolet light. The length of exposure needed to form the chemical microsensor film may be from about 1 to about 5 minutes in length. Alternatively, this step may be only from about 2 to about 4 minutes in length. In methods of the invention in which this irradiation step is carried out in an inert atmosphere, the irradiation may last longer than five minutes.
The invention of this application also includes devices for sensing a chemical that comprise a chemical microsensor film formed using the methods of the invention. As noted above, such films are formed by pretreating an oxide surface of the apparatus to form a functionalized surface, coating the functionalized surface with a prepolymer solution exposing the prepolymer film to a templating molecule, and polymerizing the prepolymer solution film with ultraviolet light to form the chemical microsensor film. Such devices may include surface acoustic wave (SAW) devices, optical waveguide devices, fiber optical devices, and field effect transistors.
The invention also includes methods for selectively attaching, or xe2x80x9cpatterningxe2x80x9d a chemical microsensor film to specific regions of an oxide surface. In some versions, this method comprises the steps of pretreating the oxide surface to form a functionalized surface, coating the functionalized surface with a prepolymer solution, exposing the prepolymer film to a templating molecule corresponding to specific sensing elements on attaching a photomask to the functionalized surface so as to leave the specific regions of the functionalized surface intended for attachment of the microsensor film exposed, and polymerizing the prepolymer solution film on the specific regions with ultraviolet light to form the chemical microsensor film. The prepolymer film that has not been exposed to radiation can then be washed away so that the process may be repeated with a different prepolymer solution and a different templating molecule to coat another sensing element. In this manner, multiple sensing elements, each with a different film having distinct recognition, binding properties, can be produced.
The invention further includes apparatuses for sensing a chemical comprising a chemical microsensor film formed using the methods of this invention. This includes apparatus including a chemical microsensor film formed by pretreating an oxide surface of the apparatus to form a functionalized surface, coating the functionalized surface with a prepolymer solution, exposing the preoplymer film to a templating molecule, attaching a photomask to the functionalized surface so as to leave specific regions of the functionalized surface intended for attachment of the chemical microsensor film exposed, and polymerizing the prepolymer solution film with ultraviolet light to form the chemical microsensor film. The apparatus of the invention may be selected from the group consisting of surface acoustic wave (SAW) devices, optical waveguide devices, fiber optical devices, and field effect transistors. Indeed, the method of patterning chemical sensing films taught in the invention is general, being applicable to all transduction approaches, of which optical planar waveguide technology is only an example.
Additionally, the invention includes apparatuses for sensing a chemical that comprise a patterned chemical microsensor film formed by pretreating an oxide surface of the apparatus to form a functionalized surface, coating the functionalized surface with a prepolymer solution, exposing the resulting prepolymer film to a templace molecule, polymerizing the prepolymer solution film or portions thereof left exposed by a photomask with ultraviolet light to form a first layer of chemical microsensor film, and forming the patterned chemical microsensor film by attaching additional layers of chemical microsensor film to regions of the oxide surface by successively repeating the previous steps. These apparatuses may be selected from the group consisting of surface acoustic wave (SAW) devices, optical waveguide devices, fiber optical devices, and field effect transistors.
The foregoing methods of the invention allow for the fabrication of covalently-attached sensing films in brief periods of time. This causes significant reductions in production costs. The methods further allow the chemical cross-linking of the polymerization reagents with the substrate surface, thus yielding a stable film, and as a result, more durable, accurate sensing surface. Further, the method allows for the fabrication of surfaces/sensor array elements with surfaces patterned with multiple types of durable, covalently-bonded sensing films for detecting multiple analytes with the same sensor element. This is accomplished by allowing the elements to be coated with different sensing films. Specifically, the method employs the process of photopolymerization to fabricate sensing films that contain molecular recognition elements and/or polymer cavities for the shape recognition of analytes in the case of molecular imprinting polymer films. Additionally, the method of the instant invention allows the fabrication of elements with multiple sensing films. Finally, the photopolymerization step of the method of the instant invention functions without the use of a photoinitiator, and results in a film that is covalently bonded to the intended surface, and which may, in many cases, be safely cleaned with a variety of organic solvents.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.